1. Field of the Invention
The present invention relates to a DC switching power supply.
2. Description of the Related Art
In DC power supplies known as "switched-mode" or "switching" power supplies, a transformer is typically employed. This transformer converts an input waveform with a given duty cycle, amplitude and frequency into a waveform with an average value approximately equal to the desired DC output voltage of the power supply. An L-C (inductive-capacitive) low-pass filter is then typically used to provide that average value as the output of the power supply. The inductor is in series with the secondary of the transformer, and the capacitor is shunted to ground. One or more electrical loads are connected to the output of the power supply.
A problem can occur when loads on the output of a switching power supply become disconnected from the supply. When this occurs, the output current from the power supply becomes reduced (or eliminated if all loads become disconnected). If the output current becomes small enough, the output voltage of the power supply can reach the peak value of the secondary voltage of the transformer of the power supply. This occurs because with a very small output current, the inductor in the L-C low-pass filter does not drop much voltage (if any at all). The capacitor in the L-C low-pass filter therefore charges up to the peak voltage of the secondary of the transformer. This peak voltage is generally considerably higher than the average voltage of the secondary of the transformer. The higher voltage which occurs across the capacitor, and therefore also at the output of the power supply, can damage components within the power supply. The higher voltage can also damage any remaining electrical loads connected to the power supply.
To prevent the voltage at the output of a power supply from rising in the manner just described, it is desirable to assure that current above a predetermined minimum is drawn from the output of the power supply at all times. One means of assuring such a minimum current draw is to connect a dummy resistor inside the power supply across the output terminals of the power supply. This resistor is sized to draw a current above the minimum necessary to prevent the output voltage of the power supply from rising. However, because this dummy resistor is always connected to the output of the power supply, the energy it consumes represents a reduction of efficiency for the power supply. Furthermore, in the event that the switching power supply is used to charge a battery, the battery will discharge through the dummy resistor when the switching power supply is turned off.
U.S. Pat. No. 3,524,124 discloses a system which switches a dummy load, a transistor, onto the output of a power supply in response to the output voltage of the supply rising above a predetermined level.
Japan Patent No. 58-64515 discloses a system which switches a dummy load, also a transistor, onto the output of a power supply if the output current from the supply drops below a predetermined level. The output current is measured with a shunt. Although this method can be effective, a shunt which can measure current of significant magnitude is a very expensive component.